Examples of edible plastified dispersions comprising a continuous fat phase and a dispersed aqueous phase are butter, margarine and related products having a lower fat content, which are often referred to as low-fat spreads. Another group of products are the so-called "melange" products, which contain both butterfat and non-dairy fat. Under normal conditions the continuous fat phase of such dispersions comprises both liquid oil and fat in the solid state. (The terms oil and fat are generally used interchangeably throughout this specification unless the context demands otherwise.)
Butter has been manufactured by a "churning" process for many hundreds of years. Margarine is a more recent phenomenon.
The starting material for making butter is dairy cream, i.e. an oil-in-water (o/w) system. Typically, butter is prepared as follows: the cream, having a fat content of about 30-50 wt.%, is pasteurised and then cooled. The cream is "ripened" for a sufficient period of time to allow solid fat to crystallise. Since milkfat has a relatively high solid fat content at low temperatures, at the end of the physical ripening, at a typical temperature of about 10.degree. C., as much as 40 wt.% of the fat may have crystallised. Subsequently, the ripened cream is churned, commonly at a temperature of about 12-15.degree. C. In the churn, the cream separates into buttermilk and aggregates of fat globules including some aqueous phase. The buttermilk is drained off and the remaining material is kneaded and shaped into butter. The buttermilk may be used for animal feed, concentrated for use in margarine production or occasionally imbibed per se by humans.
Margarine has been made in a variety of different ways. According to one such process, described in NL-A- 176 520, margarine is made starting from a margarine emulsion of the o/w type. Such emulsions are often referred to as "creams". This emulsion is very unstable, inter alia, because it contains only about 16% water, forming the continuous phase. To prevent demixing, the emulsion must be agitated during pasteurisation. Subsequently the emulsion is cooled down, e.g. by pouring it onto cooling drums. The o/w system is so unstable that spontaneous phase inversion occurs upon contact of the emulsion with the chilled surface of the drum. A similar preparation via phase inversion without separation of buttermilk, is described in DE-B- 198 508. The o/w emulsion is passed through a closed cooler. Upon cooling, the o/w emulsion inverts spontaneously to the w/o structure.
Nowadays, as described in Bailey's Industrial Oil and Fat Products, Volume 3, John Wiley & Sons, New York, (1985), pp. 77-84, margarine is commonly prepared by preparing a water-in-oil emulsion, cooling and working the emulsion to plastify it, optionally passing it through a resting tube, and packing it.
The cooling and working treatments are carried out by passing the emulsion, of which the fat is essentially completely molten, via a high-pressure positive pump, through a series of scraped-surface heat exchangers, chilled e.g. with liquid ammonia (called A-units), and stirred crystallisers having pins on a variable-speed shaft intermeshing with stationary pins on the cylinder wall (called C-units). C-units may be positioned before, in between or after the A-units in the production line.
The A-units are typically operated at shaft speeds of 300 to 700 rpm. They do not serve solely to cool the emulsion; the high internal pressures and shearing forces generated by the blades and their holding pins cause rapid crystal nucleation and further emulsification. The cooling rate in the A-units is so high that the emulsion emerging from it is undercooled. Crystallisation proceeds in the subsequent C-units, and furthermore, in the C-units, large crystals are broken up by the agitation. Such cooling and working apparatus is generally referred to as a "Votator line", "Votator" being a registered trade mark.
It will be noted from the above that the production processes of margarine and butter are markedly different.
One of the most striking product differences between typical margarines and butter is the comparative ease with which the margarines can be spread. Improvements on the spreadability of butter can be accomplished by the addition and intimate mixing of a low-melting oil, such as a vegetable oil. A product of this type, "Bregott", has been manufactured in Sweden since 1970 with increasing success. Such products, containing both butterfat and vegetable oil are known as "MELANGE" products. These products are favoured not only for their ease of use, but also for dietary and economic reasons. For example, a melange product can contain a high proportion of polyunsaturated fatty acids; the dietary preference for such acids being well established.
While the present invention will be described with particular reference to products containing both butterfat and vegetable oil, it should be noted that the invention extends inter alia to products containing butterfat and non-dairy fats in general. Thus, marine oils can replace the vegetable oil of melange products in whole or in part. Moreover, a quantity of the oil may be replaced by a suitable fat replacer, such as a sucrose-fatty acid ester. Thus the term "non-dairy fat" should be read as including fat replacer within its scope.
In the production of melange products a variety of methods have been proposed to accomplish the addition of the vegetable oil to the dairy fat. A range of methods are discussed in "Nordeuropaeisk mejeritidsskrift" N.degree. 8-9/90 p. 217. These include: addition of vegetable oil to milk before the separation of cream for buttermaking, addition of vegetable oil to cream before or after pasteurisation and reworking of butter with oil after churning.
All of these variant processing routes have a tendency to mask butter flavour with a "margarine" flavour, especially where low levels of butter on total fat are employed. This alteration of flavour has been described as "oiliness" and has resulted in the melange products scoring lower than butter in taste-trials. To some extent this problem can be overcome by the addition of flavouring agents. However, while these flavouring agents are often of natural origin, they can be expensive to produce and are generally not preferred by the consumer.
Furthermore, rather more specific difficulties have been met with the processing routes described above.
Where vegetable oils are added prior to churning, difficulties have been met in that the resulting mixture will not churn. This restricts the possible choice of vegetable oils. It has been proposed to overcome this problem by the use of additives such as purified milk proteins, lecithin and/or other emulsifiers. As with the flavouring agents discussed above, the use of these additives is not acceptable to all consumers.
Where vegetable oils are added after churning, difficulties have been met in retaining the valued texture and mouth-feel of butter, as the reworking process is often so disruptive that the characteristic structure of butter is lost.
For example, GB 1 598 362 discloses a process in which butter oil or molten whole butter is vigorously stirred with an aqueous phase prepared from skimmilk or sweet buttermilk. vegetable oil may be added to the butter or butter oil phase before addition of the aqueous phase; milk salts, such as caseinates, are added to the buttermilk prior to combination with the fat phase. During this process the structure of the butter is almost completely lost if not entirely disrupted.
Australian patent 526 763 is generally concerned with overcoming the impact of seasonal cream production on buttermaking plant operation. The patent teaches how a fat or oil may be included in butter after separation of buttermilk to prevent loss of the fat or oil in the buttermilk. The dispersion of the fat or oil through the butter is accomplished by applying shearing forces at a shear rate in excess of 50 s.sup.- 1. This can be accomplished by charging the fat or oil into the second stage of a buttermaking machine, that is the "kneading" stage. This Australian patent also discloses, in very general terms, the use of a cream to introduce the fat or oil into the butter. In each case it is considered important that "dispersal of the fat or oil through the butterfat product is effected under conditions of substantial shear action". An advantage of this process is that the buttermilk produced is of a high quality as it contains low levels of fat, minimising the fat loss into this by-product. However, the by-product is still produced in large quantities.
Some uses for buttermilk, as mentioned above, are in the manufacture of animal feeds and the preparation of margarines.
For example, SU 1 143 375 discloses a low-fat butter which contains a protein filler obtained by coagulating buttermilk. The use of buttermilk proteins is further discussed in GB 2 158 452. Jp 84/021578 discloses a process in which buttermilk is used to dilute a cream and the mixture subsequently made into butter. The preparation of a similar cream is discussed in SU 793 554.
The disadvantages of the above-mentioned art include a disparity between buttermilk production and consumption by the processes. Moreover, the processes often require complex and expensive processing equipment. Complete processing and disruptive treatment of the materials is also disclosed in EP 0 185 631.